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Earth’s climate is vulnerable to potential climate catastrophes that could threaten the longevity of civilization. Continued increases in greenhouse gas forcing could lead to the collapse of major ice sheets, which would cause catastrophic sea level rise and could cause the oceanic thermohaline circulation to halt. Further warming could cause the heat stress index to exceed survival limits, inducing hyperthermia in humans and other mammals. Even more extreme warming could shift Earth into a runaway greenhouse regime that would lead to the loss of all oceans, and the end of all life.

Geoengineering refers to the large-scale use of technology to alter Earth’s global climate, and geoengineering has been suggested as a way to ameliorate contemporary climate change. Addressing these immediate climate challenges through a combined strategy of adaptation, mitigation, and (if needed) geoengineering is a critical issue facing us today. Whether or not we decide to engage in geoengineering today, we must still devise a long-term strategy to address our changing climate.

But in the longer-term, could we also use geoengineering techniques to increase the size of the polar ice caps? In a paper published in a special issue of the journal Futures, I raise the question, “Should we geoengineer larger ice caps?” By doing so, the global average temperature of Earth could be lowered from its current state to a new stable regime with much larger ice caps. Earth has experienced shifts in ice coverage in its past, and a prolonged program of geoengineering–say, lasting a thousand years or more–could allow us to permanently shift the energy balance of Earth. More ice at the poles increases the amount of sunlight reflected back to space, leading to cooler temperatures.

Of course, the unfortunate side effects of this idea would be mass migration of populations near the poles, shifts in global agricultural zones, and a required commitment of millenia in order to avoid undesired side-effects. Human civilization today probably lacks the fortitude to embark on such a long-term goal. Nevertheless, thinking about the long-term management of our planetary system helps us realize that we have already entered the epoch of the Anthropocene. Our civilization itself is fundamentally intertwined with our global climate, and we should allow humility, rather than hubris, guide decisions to control our environment.

Global catastrophes are events that could severely cripple or destroy the foundations of civilization. Potential global catastrophes include nuclear winter, large asteroid impacts, super-volcanic eruptions, and pandemics. Humans may not necessarily become extinct under such scenarios, but, without adequate advance preparation, rebuilding civilization following such a catastrophe could prove difficult.

In a paper published in a special issue of the journal Futures, my co-authors and I present several concepts of “Isolated refuges for surviving global catastrophes.” Although catastrophic events could destroy a significant portion of the human population, isolated refuges would provide a way to protect a small group of humans so that they survive long enough to rebuild civilization. We discuss several factors that are critical for ensuring the success of a refuge, including self-sufficiency, a continuous population, secrecy, and adequate monitoring of the outside world.

We also discuss the concept of surface-independence, suggesting that an underground, underwater, or space-based refuge might provide the greatest protection of its inhabitants from the effects of global catastrophes. Any of these refuges could significantly make the human species more resilient to catastrophic threats. Space-based refuges provide an exceptional degree of isolation from Earth, and the cost of such an extraterrestrial refuge might be best “piggybacked” onto existing scientific endeavors that seek to establish a permanent presence on the moon or elsewhere in space.

Small red stars, known by astronomers as “M-dwarfs”, are the most abundant type of star in the sky and are also the most long-lived of all stars. This means there are plenty nearby of M-dwarfs to search for possible habitable planets, and many current and planned exoplanet surveys emphasize the search for potential worlds orbiting within the habitable zone of these low-mass stars. Astrobiologists often use the term “habitability” to indicate a planet’s ability to sustain liquid water on its surface, thereby providing conditions where life might be able to develop and thrive. The corresponding “habitable zone” describes the range of orbital distances that can support these clement conditions and not lose the water to a rapid runaway greenhouse (from too close an orbit) or a cool condensing atmosphere (from too far an orbit).

The problem with planets orbiting M-dwarfs is that they are prone to fall into “synchronous rotation” so that one side of the planet always faces the star, while the other side remains in perpetual darkness. Synchronous rotation can occur as a result of tidal forces from gravitational interactions between two orbiting bodies (Earth’s moon is an example of an object in synchronous rotation, so that we only ever see one side from the ground). For a planet orbiting an M-dwarf, the “sub-stellar point” beneath a constant stream of starlight is ceaselessly warmed, while the opposing “anti-stellar point” receives no starlight at all and resides in total darkness. One potential problem is that the atmosphere may condense into large ice caps on the frigid night side of these planets, which could result in total atmosphere collapse and the loss of habitable conditions.

Fortunately, the large-scale motions of the atmosphere help to redistribute this energy and, in many studies with climate models, can help avoid this atmospheric freeze-out. In a paper published in the Monthly Notices of the Royal Astronomical Society, my co-author and I use a three-dimensional computer climate model to examine the role of geothermal heating on planets orbiting M-dwarfs. Geothermal heating is another consequence of tidal forces from a close orbit, and this additional surface warming can help to amplify the asymmetric distribution of energy transport toward the night side of the planet. This can help to induce the melting of ice near the anti-stellar point and create additional habitable area surrounding the night-time hemisphere.

We also examine the large-scale dynamical circulations on these synchronous rotating planets in comparison to the general circulation patterns on Earth. We demonstrate that the direction of of the meridional (i.e. north-south) circulation changes directions from one side of the sub-stellar point to the other. That is, a global average of the meridional circulation provides an incomplete picture of the large-scale dynamics because the eastern and western hemispheres each show strong motion but in opposite directions that cancel when summed together. Additionally, we examine the presence of a cross-polar circulation that transports energy and mass from the sub-stellar to anti-stellar point across the northern and southern poles. This also contributes toward maintaining climate stability and avoiding atmospheric freeze-out with a circulation pattern atypical of those observed on Earth.

Our study emphasizes the need for careful analysis when considering how the atmospheric dynamics of a synchronously rotating terrestrial planet may differ from our own. The study of Earth-like exoplanets must begin with analogies to observations on Earth, and studies like ours help to apply Earth system models toward more general planetary system. As research into planetary habitability continues, through theory as well as observations, we will indeed continue to observe how even basic physical principles can manifest in very different ways on these alien worlds.

Geologic records over the past million years indicate cycles in the extent of Earth’s surface covered by ice. These ice age cycles are a result of variations in Earth’s orbital geometry, but it is unclear how these variations will continue in the presence of significant human emissions. In a paper published in the Journal of Advances in Modeling Earth Systems, I develop a simple climate model to demonstrate the potential for human-induced climate change to damp out variations in ice coverage, which suggests that human actions today could have long-lasting impacts into the future.

Long-term patterns in Earth’s climate show glacial cycles that correspond to variations in Earth’s orbital geometry and affect the overall amount of sunlight that the planet receives. Known as “Milankovitch cycles”, these variations are observed in geologic reconstructions of temperature and isotopes to show periodic changes every 23,000, 41,000 and 100,000 years. The first two of these correspond directly to changes in Earth’s tilt (i.e. obliquity) and wobble (i.e. precession), but the longer 100,000 year variations in orbit seem too weak in magnitude to drive the strong climate signals we observe.

One solution to this problem is that the climate system itself amplifies these small changes to create more noticeable periodic signals. These amplification mechanisms could be the large thermal intertia of the oceans, the vast energy required to move giant ice sheets, or long-term cycles in greenhouse gases such as carbon dioxide and methane. Any combination of mechanisms such as these could magnify small changes in sunlight from Milankovitch cycles and create the dominant 100,000 year cycle in ice coverage seen in the geologic record.

Studying this problem has proven to be challenging because of the long time scales involved. Most contemporary climate models are focused on patterns of climate on Earth today and in the near future of a few hundred years from now, but few modelers have focused their attention on the more distant future of climate. In my paper I develop a simple climate model that uses stochastic (i.e. randomly generated) forcing to achieve a state of resonance that displays a 100,000 year cycle in ice coverage. The model is an idealization of the more complicated Earth system and provides a tool for exploring the behavior of climate over these long time scales.

Calculations with this model show that the influence of human emissions into the atmosphere can affect the presence of the ice age cycles, either by damping the magnitude of changes or by ceasing the cycles altogether. The simplified calculations here cannot predict exactly when this should occur, but this study points toward the existence of a threshold beyond which ice age cycles may cease as a result of human emissions.

The future of Earth’s climate is becoming increasingly marked by the presence of human activity. Depending on the course of events over the next few hundred years, we may find that the damping or cessation of ice age cycles is yet another indicator of the dawning of the age of the anthropocene.

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